The present invention concerns novel telemetry system for communicating with a pacer that is implanted within a patent.
In U.S. Pat. No. 4,361,153, issued Nov. 30, 1982 and assigned to the assignee of the present invention, there is disclosed the use of a resonant impedance modulated transponder, in a device implanted in the patient, to modulate the phase of a reflected magnetic signal that is the product of a magnetic carrier imposed from outside of the body. In this manner, information is transmitted from a fixed internal implant to a positionable external telemetry unit. A relatively high energy magnetic field at a carrier frequency is established by a transmitter in the external unit. The field permeates the skin, underlying tissue and case of the implant and induces a signal in a resonant, impedance modulated transponder in the implant tuned to the carrier frequency. A second field is reradiated or reflected at the carrier frequency by the resonant transponder. The transponder's impedance is varied in accordance with a modulation input signal, causing a shift in the phase angle and amplitude of the transponder's contribution to the composite reflected signal, thereby resulting in a proportional phase and amplitude shift in the composite reflected signal. The composite reflected signal is picked up and demodulated by a phase shift detector in the external telemetry unit.
We have discovered a different system for providing communication between the device implanted in the patient and the external telemetry unit. While the system disclosed in U.S. Pat. No. 4,361,153 typically utilizes a center coil which is driven by a fixed frequency oscillator with a pair of two outer coils picking up the carrier which is phase shifted by the pacer, the system we have discovered is simpler in that it may utilize a relatively simple coil that is actually separated from the circuitry in the external telemetry unit. Thus utilizing our discovery, a separate patient's coil, which is lightweight, can be positioned externally on the patient substantially overlying an implanted pacer, and a housing containing the other components of the external telemetry unit may be separated from the patient's coil. Further, our invention has the advantage that it requires relatively few components, it is simple to manufacture, and is easy to operate.
In Schulman et al. U.S. Pat. No. 4,223,679, a telemetry system is disclosed in which an externally located oscillator is controlled by impedance changes in an impedance reflecting circuit located in an implanted pacer. We have discovered that the oscillator-controlled type of telemetry system can be improved upon significantly. For example, we have discovered that the oscillator output can be arranged to provide detection of the proximity of the patient's coil in our system using an analog varying signal having a parameter which varies in inverse proportion to the proximity of the patient's coil to the pacer. This enables the operator to determine when the patient's coil is in the proper position overlying the implanted pacer. We have also discovered the use of an automatic gain control circuit for correcting the gain from the oscillator output notwithstanding varying locations of the patient's coil with respect to the pacer. This enables the chart recording of the telemetered signal to be accurate and consistent.
Other advantages of the present invention will become apparent as the description proceeds.